A corona may be seen when thin clouds partially veil the sun or moon.
Look for one around the moon when it is near to full and the sky is
dark. When searching for a solar corona, shield
the sun and reduce the light intensity to safer levels by looking
at the sky reflected in a pool of water
or a mirror of plain glass. Staring directly at or near to the sun
can permanently damage eyesight.

Coronae have an intensely bright central aureole which is almost
white and fringed with yellows and reds. Sometimes that is all to
be seen but the better coronas have one or more successively fainter
and gently coloured soft rings surrounding the aureole. The first
ring is bluish on the inside grading through greens and yellows to
red outermost. The colours are subtle mixtures rather than the more
direct hues of the rainbow. The corona can be 15º or so in diameter
and often it shrinks and swells as different clouds scud across the
moon.

The coronae is much
smaller than the 22°
halo which can also ring the sun and moon. The corona
also has nothing to do with the Sun's outer atmosphere visible
during a total eclipse and confusingly given the same name.

Coronae are produced by the diffraction
of light by tiny cloud droplets or sometimes small ice crystals.